The use of thick-film conductors as components in hybrid microelectronic circuits is well known in the electronics field. Compositions for the manufacture of such components usually take the form of a paste-like solid-liquid dispersion, where the solid phase comprises finely divided particles of a noble metal or a noble metal alloy or mixtures thereof and an inorganic binder. The liquid vehicle for the dispersion is typically an organic liquid medium, but may also be an aqueous-based liquid medium. Additional materials may be added in small quantities (generally less than about 3% by weight of the composition) to modify the properties of the composition and these include staining agents, rheology modifiers, adhesion enhancers and sintering, modifiers.
The metals used in the preparation of thick-film conductor compositions are typically selected from silver, gold, platinium and palladium. The metal can be used either in isolation or as a mixture which forms an alloy upon firing. Common metal mixtures include platinum/gold, palladium/silver, platinum/silver, platinum/palladium/gold and platinum/palladium/silver. The most common systems used in the manufacture of heating elements are silver and silver/palladium. The inorganic binder is typically a glass or glass-forming material, such as a lead silicate, and functions as a binder both within the composition and between the composition and substrate onto which the composition is coated. Due to environmental considerations, the use of lead-containing binders is becoming less common and lead-free binders such as zinc or bismuth borosilicates are now often employed. The role of the organic medium is to disperse the particulate components and to facilitate the transfer of the composition onto the substrate.
The consistency and rheology of the composition is adjusted to the particular method of application which may comprise screen printing, brushing, dipping, extrusion, spraying and the like. Typically, screen printing is used to apply the composition. The pastes are usually applied to an inert substrate, such as an alumina, glass, ceramic, enamel, enamel-coated glass or metal substrate, to form a patterned layer. The thick-film conductor layer is normally dried and then fired, usually at temperatures between about 600 and 900° C., to volatilise or burn off the liquid vehicle and sinter or melt the inorganic binder and the metal components. Direct wet-firing, i.e. wherein the thick film layer is not dried before firing, has also been used to generate the patterned layer.
It is, of course, necessary to connect the conductive pattern to the other components of the electronic circuit, such as the power source, resistor and capacitor networks, resistors, trim potentiometers, chip resistors and chip carriers. This is generally achieved by using metal clips, typically comprising copper, which are soldered either directly adjacent to or on top of the conductive layer. Where the clips are soldered on top of the conductive layer, attachment is either directly onto the conductive pattern itself or onto a solderable composition which is overprinted onto the pattern (an “over-print”). An over-print is generally applied only in the region of the conductive pattern to which the metal clips are attached by solder, which region is generally referred to as the “clip area”. The ability to solder onto the electrically-conductive layer is an important parameter in the manufacture of heating elements since it removes the requirement for an over-print. However, the inorganic binder, which is important for binding the paste onto the substrate, can interfere with solder wetting and result in poor adhesion of the soldered metal clips to the conductive layer. The requirements of high substrate adhesion and high solderability (or adhesion of the metal clips to the conductive pattern) are often difficult to meet simultaneously. U.S. Pat. No. 5,518,663 provides one solution to this problem by incorporating into the composition a crystalline material from the feldspar family.
An important application of patterned electrically-conductive layers is in the automobile industry, and particularly in the manufacture of windows which can be defrosted and/or demisted by an electrically-conductive grid permanently attached to the window and capable of producing heat when powered by a voltage source. In order for the window to defrost quickly, the circuit must be capable of supplying large amounts of power from a low voltage power source, typically 12 volts. For such power sources the resistivity requirement of the conductive pattern is generally in the range of from about 2 to about 5 μΩ cm (5 mΩ/□ at 10 μm after firing). This requirement is readily met by conductors containing noble metals, particularly silver which is the most commonly-used material for this application.
In certain applications, a conductive composition having a higher resistivity is required. In particular, it is anticipated that the resistance requirements of window-heating elements in automobiles will shortly need to change since the automotive industry is expected to adopt the use of a 42 and 48 volt power supply in the near future. As a result, the conductive composition used to manufacture the window-heating elements will be required to exhibit higher values of resistivity, typically greater than about 10 μΩ cm, preferably greater than about 12 μΩ cm, particularly in the range from about 20 to about 70 μΩ cm.
A number of different materials may be added to adjust the specific resistivity of a conductive composition. For example, metal resinates such as rhodium and manganese resinates have been used to increase resistivity, as disclosed in U.S. Pat. No. 5,162,062 and U.S. Pat. No. 5,378,408. In addition, an increase in the content of precious metals, particularly the platinum group metals such as platinum and palladium, has also been used to increase the specific resistivity. Silver/palladium and silver/platinum compositions can achieve resistivity values from about 2 μΩ cm (that of a composition comprising only silver and binder) up to around 100 μΩ cm (for a 70:30 Pd:Ag blend). Systems comprising platinum and/or palladium are, however, significantly more expensive and their use would be prohibitive in applications requiring coverage of a large surface area, such as the window-heating elements used in the automotive industry. In addition, an over-print of a composition containing a high amount of silver (and typically small amounts of filler) is generally required for certain metal blends, such as compositions containing high palladium levels, in order to achieve adequate solder adhesion. Conventional conductive compositions which typically operate at resistivity values of 2 to 5 μΩ cm and which are comprised predominantly of silver do not require an over-print since acceptable levels of solder adhesion can be achieved by adjusting the levels of inorganic binder.
Other, lower-cost approaches for achieving a high resistivity involve blending large amounts of filler into a silver-containing conductive composition to block the conductive path. Fillers are typically inorganic materials and those commonly used are glass (which may be the same or different as that used for the binder) and alumina (or other metal oxides). However, such approaches tend to result in a loss of solder acceptance and solder adhesion. For example, adequate solder adhesion can be maintained only up to a level of about 10% alumina by weight of the composition but this level is generally too low for an appreciable rise in resisitivity. For glass-type fillers, loss of solder adhesion occurs at even lower levels and, again, this level is too low for an appreciable rise in resistivity. In addition, this problem can not normally be ameliorated by the use of silver over-prints owing to glass migration between the layers during firing, specifically from the conductive coating into the over-print.
A further advantageous property of the conductor compositions is chemical durability and resilience to exposure to varying environmental conditions such as temperature, humidity, acid and salt. Compositions comprising large amounts of glass filler, particularly lead-free glass filler, are often relatively unstable to such factors.
An additional consideration is that it is desirable for the resistance of the coating composition to be substantially independent of the temperature of firing used in the manufacture of the patterned conductive layer. For instance, in the case of the application of a conductive composition to a glass substrate, the behaviour of the composition under sintering and melting should remain substantially constant between the temperatures of about 620 and 680° C. Nevertheless, a change in resistance of up to about 10% between these two temperatures, which corresponds to the behaviour of a pure silver composition, is generally tolerated. The use of large amounts of filler to significantly increase resistivity results in compositions which do not generally satisfy this requirement.
A further additional consideration is that it is desirable for the relationship between the resistivity and the amount of resistivity modifier added to the composition to be relatively predictable and/or substantially linear within the target range of desired resistivities. The resistivity of compositions comprising large amounts of filler generally increases in an almost linear manner until a critical concentration is reached. At this critical concentration, the resistivity may rise very rapidly, often by an order of magnitude, when the level of resistivity modifier is increased by only a fraction of a weight percent. As a result, it is difficult to target specific values of resistivity for such compositions.
It is an object of this invention to provide higher-resistivity electrically-conductive compositions which do not suffer from the aforementioned disadvantages. In particular, it is an object of this invention to provide an economical electrically-conductive coating composition having increased resistivity while at the same time exhibiting good solderability.